Novel Brownfield-Rejuvenation Approach Uses Near-Wellbore-Saturation Monitoring
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Fields in the Upper Assam-Arakan Basin have been studied intensely to find prospective sweet spots, perforation intervals for new wells, and potential workover candidates. These forecasts, guided only by dynamic-numerical-model results, have had mixed results when implemented in the field. A validation of the dynamic-model forecasts with near-wellbore-saturation logs can help reduce uncertainty. The validation was carried out in old wells, which helped in making informed decisions about tapping bypassed hydrocarbon pockets.
An integrated seismic-to-simulation study was conducted on 42 reservoirs in 375 km2 of the Upper Assam Basin. As a result of this study, the overall structural framework, well correlations, and fault mapping have been revised considerably compared with previous interpretations. Previously, the only data available to manage these fields were from paper-based 2D maps, from which some volumes were estimated by use of simplistic techniques. Now, the numerical-model-based approach can provide a better understanding of stock-tank oil initially in place and leftover pockets of oil. A phased field-development plan (FDP) for the next 5–10 years was proposed from this study.
After completion of the FDP study, executing the recommendations in a timely manner was essential and the dynamic model was updated with the production results accordingly. Therefore, a work flow was created to increase the effectiveness of workover activities by incorporating validation with near-wellbore-saturation-log readings.
Using the dynamic model, the following eight factors were considered for each candidate well before a workover was recommended:
- Remaining mobile oil and pressure distribution at the end of history
- Performance of offset wells
- Quality of history match around the wells
- Porosity, permeability, and rock type
- Connectivity around the well
- Distance from gas/oil contact and free-water level
- Location of faults
- Hydrocarbon pore volume at the end of the prediction period
A method was implemented to validate the near-wellbore hydrocarbon saturations predicted in the history-matched dynamic model though integration of FDP results with pulsed-neutron-tool-log interpretations for the recommended workover candidates. Perforations were then proposed for the validated intervals. The study also aimed to eliminate preplanned workovers that were not predicted as prospective by the FDP after confirmation with near-wellbore-saturation logs.
The Work Flow
The workovers recommended by the FDP study were identified so as to incorporate sufficient hydrocarbon pore volume deemed as prospective for zone transfer or recompletion workover. The recommended workovers were checked for whether they were a part of an already prepared workover list. If so, a pulsed-neutron-tool log was recorded to interpret the near-wellbore saturation. The purpose of running the pulsed-neutron tool was to validate the FDP-study result with actual log data. If the saturations from the two sources matched within a reasonable range, zone transfer or recompletion was recommended. The production results obtained from workover were used to update the dynamic model.
If the proposed workover was not in the recommendation list of the FDP study, it was checked for any upside potential. Once any upside potential was identified, an uncertainty analysis was run, taking into account offset-well data, limited acquired data, similar field-production trends, and petrophysical uncertainty to estimate the range of saturations expected for the candidate. Thereafter, a pulsed-neutron tool was run through the well and the interpreted saturation was checked to ascertain whether it was within the previously determined prospective range. If saturation was found to be within the range, the interval was suggested for perforation or the well was removed from the workover plan and the results were incorporated in the model.
If the proposed workover was not in the FDP recommended list or showed no upside potential, the saturation from the pulsed-neutron-tool log in that well was compared with the model saturation. If both suggested a pessimistic scenario, the well was removed from the workover plan. If the pulsed-neutron tool suggested a prospective scenario in a particular well, the results were analyzed considering the dynamic-model response and a decision was made whether to perforate the zone. The production result, if the well were perforated, was then incorporated in the model.
This robust exercise ensured that the workovers yielded maximum incremental hydrocarbon gain with very short rig time.
Examples of FDP-Study Validation With Pulsed-Neutron Tool
Well A was considered for zone transfer in an arenaceous formation. The well was on the list of FDP-recommended workovers on the basis of the hydrocarbon-pore-volume analysis at end of history and, thus, was recommended for a pulsed-neutron-tool run. The saturation obtained by interpretation of the pulsed-neutron-tool run in the well matched closely with model predictions (Fig. 1 above), and the interval was recommended for perforation. After perforation, the interval produced at the recommended rate.
Well B, which was on the workover list but not recommended for workover by the FDP study, lies in an argillaceous formation but experienced sand production. The interval considered for workover has low oil saturation, which was validated by the pulsed-neutron tool.
Well C is one of the wells for which the results from the pulsed-neutron test did not match closely with the model predictions. The pulsed-neutron recording suggested fair oil saturation around the well; however, the model predictions suggested insignificant oil saturation, unfeasible for exploitation by workover. Upon perforation in the interval, however, the well produced oil, corroborating the pulsed-neutron-test results. The post-perforation results were then suggested to be used as a guide for model updating. Updating the model can enhance the precision of the model.
The benefits of this work flow included the following:
- This work flow is established as a routine exercise for rejuvenation of old wells, some of which were otherwise abandoned. It also helped avoid unproductive workovers, and the incorporation of workover results enhanced predictability of the dynamic model, which can be used for wider field-development planning.
- The work flow enhanced confidence in the dynamic model, which thereafter was used to drive development plans to increase hydrocarbon production incrementally.
- The work flow played an important role in making the critical decisions for future hydraulic fracturing in these 50- to 60-year-old wells, which were shut in for more than 20 years. Post-fracturing hydrocarbon-production results validated and increased the confidence in the pulsed-neutron-tool results. The pulsed-neutron-tool results were very important data, not only for the operator but also for designing the fracturing job.
- Some fields in the Upper Assam Basin have severe sand-production problems. The sand production increased the difficulty of producing hydrocarbon from the wells because of sand accumulation in the wells and in surface lines. A decision was made to instigate gravel-pack operations in the sand-producing wells. This work flow helped in ascertaining the sand interval for accurate gravel-pack placement.
- A few radial-drilling candidates were identified where the work flow was used to identify the oil- and gas-bearing zones so radial drilling could be performed at the same depth. This resulted in increased oil and gas production in such wells. Now, pulsed-neutron-tool-data acquisition has become mandatory for these selected candidates.
- Executing this work flow reduced rig time for production and testing by 60%.
Previously, significant time had been spent on production testing during workovers and perforation depth was decided on the basis of 50- to 60-year-old openhole logs. Much ambiguity existed in deciding the current oil/water contact and gas/water contacts because of changing reservoir dynamics and low resistivity contrast between shale and sands. The testing performed with reference to the openhole logs used to result in mostly water production. Also, many hydrocarbon-bearing sweet spots within the sand had been left unexploited because of unknown current reservoir behavior. The new work flow, using recommendations of a sound FDP study, validated by a pulsed-neutron tool, proved to be the solution for enhancing hydrocarbon production.
Novel Brownfield-Rejuvenation Approach Uses Near-Wellbore-Saturation Monitoring
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